In present, conventional printed circuit boards for use in electronic circuitry, the electrical conductors or trace are basically two dimensional in space. A trace on a board of this type is a conductor applied to the upper surface of a substrate, the length and width of the trace being in the x and y directions and the thickness (which is extremely small) being is in the z axis perpendicular to but extending upwardly from the upper surface of the substrate. Electronic components, such as resistors, capacitors and the like, are also placed on the surface of the substrate. Such components typically have lead wires which are coupled to respective traces by inserting the leads into holes which intersect appropriate circuit traces. The leads are attached mechanically and electrically by soldering. If the components are leadless components, they are mounted on the upper surface of a substrate in briding relationhip to respective traces. Such leadless components are held in place temporarily by solder paste or by an adhesive until they are rigidly soldered to the traces to form a permanent bond. This solder joint then serves as a structural attachment and as an electrical joint.
The usual technique for interconnecting electrical components and traces on a conventional printed circuit board is as follows:
1. The various electrically conductive surface traces are provided on the surface of a dielectric substrate. This can be done by using conventional printed circuit board production methods. PA1 2. The components are attached to the substrate in a predetermined arrangement. PA1 3. The terminations of the electrical components are joined to the circuit traces by means of soldering. Typically this is by wave solder (high temperature) at approximately 500.degree. F. or by reflow solder of solder posters. This requires that the substrate remain stable and not exhibit damage or deterioration during or after this cycle. Also, the soldering process requires 100% inspection, repair, and reinspection. PA1 1. The invention permits the electrical traces of a circuit to be densely packed to thereby minimize the size of a circuit board without sacrificing the reliability of the circuit. PA1 2. The invention eliminates the need for wire bonding of leads to semiconductor chips. PA1 3. The invention provides a circuit board having fewer electrical connections but connections of much higher structural integrity than is found in conventional circuit boards. PA1 4. The invention permits circuit boards to be produced at lower costs compared with the costs of conventional circuit boards. PA1 5. The invention may permit improved dissipation of heat from semiconductor chips because of the improved access to the rear surfaces of such chips. PA1 6. The present invention is also adaptable for design and manufacture of discrete devices, such as integrated circuit packages with terminal leads for insertion into printed wiring boards or other circuit boards.
The conventional method of assembling an integrated (semiconductor) chip is by way of bonding gold wires from metallized terminal pads on the chip to a lead frame by use of a thermal bonding machine or an ultrasonic bonding machine. After bonding of the gold wire from the pads to the lead frame, the chip is encapsulated or molded into an integral form.
All of the foregoing techniques in the production of conventional printed circuit boards and integrated circuit chips are time-consuming and relatively expensive. Moreover, a circuit board made in accordance with conventional techniques has electrical traces which take up considerable space on the surface of the substrate so that the surface of the substrate itself must be relatively large to accommodate a large number of traces and components and to provide for traces of sufficient size to carry the signal current necessary to assure proper operation of the circuitry on the substrate.
Since the traces are on the upper surface of the substrate, the traces must have sufficient width to minimize the electrical resistance of the current path defined by the trace. To achieve this end, considerable space is used on the substrate surface. This limitation requires relatively large substrates for relatively complex circuitry, thereby increasing the cost of producing a printed circuit board as well as for assembling the components on the circuit board to form a circuit thereon.
In view of the foregoing limitations of conventional printed circuit boards, a need exists for improved circuit board which avoids such limitations.